This invention relates generally to magnetic disk drive units and more particularly to such drive units for accommodating both a user-removable disk pack and a field-replaceable fixed disk assembly.
Many different models of magnetic disk drive units are known in the art. Several of these models accommodate a removable disk pack, which contains one or more magnetic disks mounted on a common hub, for example of the IBM 3336 type. This type of disk pack is intended for ready interchange between different drive units and is typically used to store large data bases which are generally also recorded on some other medium such as a back-up magnetic tape. Such disk packs usually dedicate one disk surface for storage of servo track information. That is, the dedicated disk servo surface has track information recorded thereon at the factory by a precision track writing device. In use, a read/write head positioner within the drive unit responds to error information derived from the servo surface head to precisely position that head adjacent a servo track. The drive unit contains a plurality of read/write heads, each uniquely associated with a different disk surface, which heads are physically connected to the servo track head so that the positioning of the servo track head correspondingly positions the other heads.
Some disk drive models also contain one or more nonremovable of fixed disks in addition to accommodating a removable disk pack. The fixed disks are installed by the drive unit manufacturer and are neither removable by a user operator nor field replaceable by a service technician. The reason that the fixed disks are not field replaceable is that great precision is required to assure retention of concentricity between prerecorded servo tracks and the center of rotation of the drive unit drive spindle. Due to the precision required, it has been necessary to return the entire drive unit to the manufacturer or some suitable service facility having a precision track writing capability in order to replace fixed disks.